Electrochemical devices comprising a non-aqueous electrolyte have been used in a wide range of electric and electronic appliances. Such electrochemical devices include, for example, non-aqueous electrolyte batteries such as a lithium battery, and capacitors such as an electric double layer capacitor and an electrolytic capacitor.
It is desirable that a non-aqueous electrolyte comprising a non-aqueous solvent and a solute has a high ionic conductivity. In order to obtain such a non-aqueous electrolyte, a non-aqueous solvent having a high dielectric constant and a low viscosity is required. However, a non-aqueous solvent having a high dielectric constant usually has a strong polarity and therefore has a high viscosity. Thus, a mixed non-aqueous electrolyte containing a non-aqueous solvent with a high dielectric constant such as ethylene carbonate (dielectric constant: 90) and a non-aqueous solvent with a low dielectric constant such as dimethyl carbonate (dielectric constant: 3.1) or ethyl methyl carbonate (dielectric constant: 2.9) is commonly used.
As the solute constituting the non-aqueous electrolyte, for example, LiPF6, LiClO4, LiBF4, LiAlCl4, LiSbF6, LiSCN, LiCl, LiAsF6, LiCF3SO3, (CF3SO2)2NLi or the like is used. Among them, LiPF6 is most frequently used because it has a wide potential window and thus provides a non-aqueous electrolyte having a high ionic conductivity.
While a non-aqueous electrolyte containing LiPF6 has a high ionic conductivity of approximately 8.5 mS/cm at room temperature, it has a low thermal stability. Moreover, LiPF6 is problematic in that it reacts sharply with moisture to decompose. HF, PF5 or the like produced by the decomposition of LiPF6 has an adverse effect on an electrochemical device. Accordingly, when an electrochemical device, which comprises a non-aqueous electrolyte containing LiPF6, is used for an appliance generating a great amount of heat, the performance of the electrochemical device may deteriorate or a gas may be generated inside the device.
Although the uses of LiSO3CF3, (CF3SO2)2NLi and the like have also been investigated, they are not proceeding towards practical utilization. This is because many of solutes containing an organic anion tend to cause a corrosion of a current collector made of aluminum, which is used in the positive electrode of a non-aqueous electrolyte battery.
On the other hand, LiBF4, which is commonly used in lithium primary batteries, has a higher thermal stability as compared with LiPF6 and a potential window substantially equal to that of LiPF6. However, a non-aqueous electrolyte obtained by dissolving LiBF4 in a non-aqueous solvent has a low ionic conductivity of about 2.9 mS/cm at room temperature. For this reason, LiBF4 is not used in lithium secondary batteries.
The reason why LiBF4 has a lower ionic conductivity as compared with LiPF6 is that only four fluorine atoms having an electron withdrawing property are bound to one boron atom in LiBF4, whereas six fluorine atoms are bound to one phosphorus atom in LiPF6. Since electronegativity depends on the number of the bound fluorine atoms, the electronegativity of the BF4 anion is lower than that of the PF6 anion. Therefore, it is considered that LiBF4 is difficult to dissociate into a Li cation and a BF4 anion. Moreover, because the BF4 anion has a small ionic diameter, it tends to cause an association thereby to deteriorate the ionic conductivity of the electrolyte.
The dielectric loss in an electrolytic capacitor is greatly affected by the ionic conductivity of the non-aqueous electrolyte interposed between a negative electrode foil and a dielectric layer which comprises an oxide of aluminum or tantalum disposed on a positive electrode foil. When the dielectric loss is great, the frequency characteristics and the charge/discharge characteristics of the electrolytic capacitor are deteriorated. Nevertheless, a non-aqueous electrolyte accommodated in an electrolytic capacitor contains a solute which dissociates to produce a BF4 anion, resulting in a great dielectric loss in the capacitor.